It was realized early in ultrasonic transport phenomena that particles with different mechanical properties (density and compressibility) can be separated in a solution. Particles in an ultrasonic standing wave field that are more dense and less compressible than the background medium are transported to a different spatial location than particles that are less dense and more compressible than the background medium, producing a true binary separator based upon mechanical properties. Ultrasonic radiation pressure has been demonstrated as a viable means to manipulate and locally trap particles in microfluidic environments. In application, the acoustic radiation pressure force depends upon frequency of excitation, pressure amplitude within the medium, and the density/compressibility contrast between the particle of interest and the host medium. Within an applied ultrasonic standing wave, particles experience a drift force resulting from acoustic radiation pressure that transports the particles to a position within the applied standing wave that corresponds to minima in the acoustic radiation pressure force potential. In general, the location of this minima is located in a different spatial location for particles that are more dense and less compressible than the background medium in comparison to particles that are less dense and more compressible. The technique has been successfully demonstrated in particle conditioning experiments involving trajectory steering, agglomeration, retainment, mixing, selective retainment and deposition of cells on a surface.
Separation utilizing acoustic radiation pressure has not been readily implemented in biological problems since most biological particles of interest (e.g. red blood cells, white blood cells, bacteria, spores, etc.) all are more dense and less compressible than water. Recent biological assays have implemented means to separate analytes from sample solutions by taking advantage of their physical properties. For instance, positively charged silica surfaces are used to capture highly negative charged DNA molecules from complex samples. More recently, capture particles have been employed to capture biological particles of interest. This has been especially true for magnetic field-base purification methods where magnetic particles are used to capture particles of interest from within a sample and then separated from the sample using magnetic fields.
It is desirable, therefore, to provide an apparatus and method for separating particles within a medium utilizing engineered capture particles and acoustic radiation pressure.